a) Field of the Invention
This invention relates to a real image mode variable magnification finder optical system for use in photographic cameras, video cameras, etc.
b) Description of the Prior Art
For the finder optical system in which a photographing system is disposed separate from a finder system, an inverse Galilean filter is well known. This finder optical system, however, has drawbacks that its field frame is unclearly viewed and the visual field itself cannot be sufficiently observed due to ghost and flare produced at a half mirror for forming the field frame.
In contrast to this, a Keplerian finder optical system is such that since a real image formed by an objective lens system is observed through an eyepiece system, the drawbacks of the inverse Galilean finder optical system are practically eliminated and the finder with a good view is brought about. A case where the Keplerian, namely, real image mode finder optical system possesses a function of varying magnification is shown in FIG. 1, which uses two units of zoom lenses as an objective lens system 1 comprising a first negative lens unit 2 and a second positive lens unit 3. The finder optical systems of the type are known from, for example, ones set forth in Japanese Patent Preliminary Publication Nos. Sho 61-156018, Sho 64-65519, and Hei 1-309020. In a real image mode variable magnification finder optical system, although an erect image system, such as a Porro prism or a pentagonal prism, is used to convert an observation image from an inverted image to an erect image, a method is known from the past that the reflecting member of the erect image system is divided and a part of the divided reflecting members is moved to the back focus of the objective lens system ranging from the last lens surface to the intermediate imaging position of the objective lens system to thereby include an intermediate image in the erect image system for reduction of the overall length of the finder optical system. To change the inverted image to the erect image, at least four-fold reflection is required, but the two-unit zoom lens type finder of negative and positive powers shown in FIG. 1 has a power arrangement of a retrofocus type, so that it is possible to dispose the reflecting member for once- or twice-reflection even at a low magnification position (wide angle) where the back focus is minimized.
In such a finder system, however, the refracting power of each lens unit must be increased in order to make shorter the overall length of the objective lens system while ensuring a long back focus in which the reflecting member can be disposed, with the resultant problem of increasing the yield of aberration in each lens unit. It is difficult that since spherical aberration and astigmatism produced at the second lens unit having positive power for varying magnification each increase on the minus side in particular, plus aberration is produced only by the first lens unit having negative refracting power for correcting a diopter shift caused by the magnification change, thereby improving the performance of the entire objective lens system.